Evaluations of single walled carbon nanotubes using resonance Raman spectroscopy

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 77-81).This work reports the results of two studies which use resonance Raman scattering to evaluate the vibrational properties of single walled carbon nanotubes (SWNTs). In the first study, we report an evaluation of second-order combination and over-tone modes in highly ordered pyrolytic graphite (HOPG), in SWNT bundles, and in isolated SWNTs. We found both dispersive and non-dispersive Raman bands in the range 1650-2100 cm⁻¹, and we show that the appearance and frequency vs. laser energy E[laser] behavior of these features are in agreement with predictions from double resonance Raman theory. In the case of SWNTs, these second-order bands depend on the one-dimensional structure of SWNTs, and, at the single nanotube level, the spectra vary from tube to tube, depending on tube diameter and chirality, and on the energy of the van Hove singularity relative to E[laser]. In the second study, we present a theoretical method of predicting, to within a linear constant [beta], the frequency shift in the Raman features of a SWNT material as the Fermi level is changed by depletion or addition of electrons. This constant is then evaluated for different Raman modes in SWNTs by comparing theoretical predictions to experimental observations by Corio et al. , where the Fermi level of SWNT bundles is raised by electrochemical doping and Raman spectra are collected in situ. It is determined that for the G-band of SWNTs, the dependence of frequency on Fermi energy is /[beta][sub]G = 271cm⁻¹ per hole per C-atom for metallic SWNTs with d[sub]f [approximately]= 1.25 ± 0.20nm.by Victor W. Brar.S.B

Electrostatic Steering of Thermal Emission with Active Metasurface Control of Delocalized Modes

We theoretically describe and experimentally demonstrate a graphene-integrated metasurface structure that enables electrically-tunable directional control of thermal emission. This device consists of a dielectric slab that acts as a Fabry-Perot (F-P) resonator supporting long-range delocalized modes bounded on one side by an electrostatically tunable metal-graphene metasurface. By varying the Fermi level of the graphene, the accumulated phase of the F-P mode is shifted, which changes the direction of absorption and emission at a fixed frequency. We directly measure the frequency- and angle-dependent emissivity of the thermal emission from a fabricated device heated to 250$^{\circ}$. Our results show that electrostatic control allows the thermal emission at 6.61 $\mu$m to be continuously steered over 16$^{\circ}$, with a peak emissivity maintained above 0.9. We analyze the dynamic behavior of the thermal emission steerer theoretically using a Fano interference model, and use the model to design optimized thermal steerer structures.Comment: 8 pages, 4 figure

The Effect of Dust and Hotspots on the Thermal Stability of Laser Sails

Laser sails propelled by gigawatt-scale ground-based laser arrays have the potential to reach relativistic speeds, traversing the solar system in hours and reaching nearby stars in years. Here, we describe the danger interplanetary dust poses to the survival of a laser sail during its acceleration phase. We show through multi-physics simulations how localized heating from a single optically absorbing dust particle on the sail can initiate a thermal-runaway process that rapidly spreads and destroys the entire sail. We explore potential mitigation strategies, including increasing the in-plane thermal conductivity of the sail to reduce the peak temperature at hotspots and isolating the absorptive regions of the sail which can burn away individually.Comment: 7 pages, 4 figure

Scanning Tunneling Spectroscopy of Graphene and Magnetic Nanostructures

This dissertation is divided into two parts, both of which describe measurements where a scanning tunneling microscope (STM) is operated to perform measurements of novel physical systems. In the first section I describe spin-polarized STM (SP-STM) measurements on individual adatoms on a ferromagnetic surface. The aim of the first section is to illustrate how a SP-STM can be used to probe magnetic phenomena at the atomic scale. After describing the explicit details of the SP-STM apparatus, I explain how to use SP-STM to study the magnetic coupling of different 3d adatoms (Fe, Cr and Cu) to a ferromagnetic surface. It is shown that the SP-STM is capable of distinguishing which species couple ferromagnetically vs. anti-ferromagnetically to the surface. The second section describes STM measurements performed on a graphene surface equipped with a back-gate electrode that can be used to vary the charge density of the graphene in situ. These measurements represent the first time an STM has been capable of measuring a gate-tunable surface, and this section describes the extra experimental and theoretical considerations that are required to set up such an experiment and accurately interpret the data. It is shown that electrons tunneling into graphene include a strong, phononmediated inelastic signal, and that an STM tip can be an invasive probe of graphene. After showing how to account for these effects, STM experiments are presented that examine (1) how impurities effect the large scale electronic structure of graphene, (2) how the quasiparticle lifetime in graphene depends on charge density, and (3) how Co adatoms can be reversibly ionized on graphene through the use of a back-gate electrode

Infrastructure quality in deregulated industries Is there an underinvestment problem?

Includes bibliographical references. Title from cover. Also available via the InternetAvailable from British Library Document Supply Centre- DSC:3597. 9512(no 3836) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Effects of pre-operative isolation on postoperative pulmonary complications after elective surgery: an international prospective cohort study an international prospective cohort study

We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care. We aimed to determine the impact of pre-operative isolation on postoperative pulmonary complications after elective surgery during the global SARS-CoV-2 pandemic. We performed an international prospective cohort study including patients undergoing elective surgery in October 2020. Isolation was defined as the period before surgery during which patients did not leave their house or receive visitors from outside their household. The primary outcome was postoperative pulmonary complications, adjusted in multivariable models for measured confounders. Pre-defined sub-group analyses were performed for the primary outcome. A total of 96,454 patients from 114 countries were included and overall, 26,948 (27.9%) patients isolated before surgery. Postoperative pulmonary complications were recorded in 1947 (2.0%) patients of which 227 (11.7%) were associated with SARS-CoV-2 infection. Patients who isolated pre-operatively were older, had more respiratory comorbidities and were more commonly from areas of high SARS-CoV-2 incidence and high-income countries. Although the overall rates of postoperative pulmonary complications were similar in those that isolated and those that did not (2.1% vs 2.0%, respectively), isolation was associated with higher rates of postoperative pulmonary complications after adjustment (adjusted OR 1.20, 95%CI 1.05–1.36, p = 0.005). Sensitivity analyses revealed no further differences when patients were categorised by: pre-operative testing; use of COVID-19-free pathways; or community SARS-CoV-2 prevalence. The rate of postoperative pulmonary complications increased with periods of isolation longer than 3 days, with an OR (95%CI) at 4–7 days or ≥ 8 days of 1.25 (1.04–1.48), p = 0.015 and 1.31 (1.11–1.55), p = 0.001, respectively. Isolation before elective surgery might be associated with a small but clinically important increased risk of postoperative pulmonary complications. Longer periods of isolation showed no reduction in the risk of postoperative pulmonary complications. These findings have significant implications for global provision of elective surgical care